I wrote an article about that. Depression and Childhood Abuse
But when you are deeply depressed and you don't know the cause, you need to look at your past. It could be because of unresolved/unforgiven issues that you've forced yourself to stop thinking about, but their effects are still there because you didn't know how to cope with them.
Also, when women get in their 30s and 40s, if they were sexually abused as children, they sometimes begin to have flashes of memories come forth. Sometimes the depression is there before the memories surface.
I'm not saying that's your problem. Just saying that sometimes it takes a therapist to help you find the root of what is causing your depression, and then some people do benefit from medications--good luck with that.